Before cloud storage services established themselves, online backup services had been around for years. Carbonite was big enough to get itself preloaded on many OEM Windows systems and became the most famous of the online services. Mozy is also very well known. At the same time, many general security suites have for years included online backup as a feature. Typically you get a few gigabytes with the base subscription price of the suite and can buy more storage. Symantec, McAfee and Comodo are three that take this approach.

As a general rule, online backup services allow you to back up and restore files. That's it. With hard drive sizes well into the hundreds of gigabytes, this necessarily means that you're only backing up a portion of your system, so many of these services default to backing up your My Documents and other data folders.

Cloud storage is different. These services take a portion of your hard drive and synchronize it with the online storage. Make changes on the hard disk and they are quickly replicated to the cloud storage. If you have other systems on the account, the files are synchronized down to them. And so as you work on your files they are automatically backed up.

So why bother with backup services at all? Perhaps a cloud service is at least as good at backup and gives you full access as well. This is very often true, and so I'm sure there are many users who are paying for an online backup service who would be better-served by a cloud storage service.

On the other hand, SugarSync is expensive. Looking at 100 GB as an example, SugarSync costs 50% more than Dropbox, 150% more than Google Drive, and 200% more than SkyDrive. Pure backup products are cheaper per gigabyte and they'd better be, because they do so much less.

I found this review roundup of 27 online backup services on About.com. A quick survey of these services gives me the impression that the backup-only ones tend to be cheaper per gigabyte, perhaps even advertising unlimited backup space.

Claims of "unlimited" service on the Internet usually have some sort of caveat in them. Many years ago I challenged Web hosting services that claimed "unlimited bandwidth" for cheap hosting accounts whether that was literally true -- and they generally admitted that customers who abuse the privilege would be persuaded to be reasonable. Carbonite says only that "...for exceptionally large backups -- 200GB or more -- backup speed will slow noticeably after the first 200GBs have been backed up."

The real question you should ask yourself is whether online backup of more than 100 GB really makes sense for you. It's possible that you have a large music or image or video library you'd like to back up; if so, a pure backup service is probably your best solution. But maybe not your only solution.

As I recently explained, Dropbox has changed the way I work because of how it keeps all my computers in synch. I don't care about backing up files anymore. They're all in Dropbox. Most of my photos are on Phanfare, a separate service I like. This is something else to consider: Are all your images in Picasso, your music in iTunes, and so on? You might already be backed up.

Paying for and using a backup service is the sign of a smart user who knows things can go wrong. Go a little further and make sure you're getting the best kind of backup for the way you use your data.